Polyurethanes with an ionic nature, which include the anionic polyurethanes, are very old in the art. Starting materials, which contain groups capable of salt formation or which already contain salt groups, can be used in the production of this type of polyurethanes. These salt groups or groups capable of salt formation can be present both in the chain lengthening agent and in the polyisocyanate or in the polyhyrdroxy compound, e.g., a polyester, employed in the first stage of the production process.
Anionic polyurethanes are produced by additionally using compounds for the synthesis of the polyurethanes so that the final polyurethane contains an acid group, e.g., the carboxyl group. By neutralization with a suitable base, the polyurethane is converted into a salt; the polyurethane molecule then exists as an anion. It is naturally also possible even at the outset to use suitable starting materials in the form of a salt in the synthesis of anionic polyurethanes. Defined as anionic polyurethanes within the scope of the invention are also such polyurethanes in which the polyurethane molecule also has a cationic moiety, in addition to the anionic part, i.e., it possesses a more or less amphoteric nature.
Numerous processes for the production of anionic polyurethanes are already known. Thus, for example, Unexamined West German Patent Application No. DE-OS No. 1,495,847 discloses a process in which rather high molecular weight polyhydroxy compounds such as polyesters, polyacetals, polyethers, etc., enter into a reaction with polyisocyanates, and whereby a compound with at least one hydrogen atom reacting with isocyanate groups and at least one salt-like group or a group capable of salt formation is used. In this case, salt groups include groups which are derived both from sulfonic acids and also from carboxylic acids and acids of phosphorus. The numerous such compounds include, for example, lactic acid, tartaric acid, 4,6-dihydroxyisophthalic acid, uric acid, barbituric acid, oxalic acid, malonic acid, diglycolic acid, aminosulfonic acids, phosphinic acids, phosphonous acids, phosphonic acids, and phosphoric acids.
The polyurethanes described therein can be processed into films and sheets and serve to coat and impregnate woven and nonwoven textiles, leather, paper, and the like, as well as to provide an antistatic and wrinkle-resistant finish. These polyurethanes are less suitable as a sizing agent for paper, because they produce high Cobb values and low degrees of sizing.
Anionic polyurethanes of similar structure, which differ from the abovementioned polyurethanes in the use of very special diaminocarboxylic acids as salt-forming compounds, are disclosed in Unexamined West German Patent Application No. DE-OS 1,720,639. Diaminocarboxylic acids are used here in which the carboxylic acid group is bound via an amide bond to the chain, which carries both amino groups. These anionic polyurethanes as well are little suitable as sizing agents for paper.
West German Patent Specification No. DE-PS 2,457,972 discloses anionic polyurethanes, which are obtained by reaction of an aliphatic dihydroxy compound, which has an aliphatic substituent with at least 10 carbon atoms, with a polyisocyanate to a prepolymer with an NCO end group, by chain lengthening of the resulting prepolymer with an aliphatic diol, which carries an acid group capable of salt formation, and by total or partial conversion of the acid group into a salt by reaction with a base or by chain lengthening of the prepolymer with an aliphatic diol, which already carries a suitable salt group. It is also possible to react the prepolymer with NCO groups first with an aliphatic trihydroxy compound in a molar ratio of about 1:1 and then with a cyclic anhydride of a dicarboxylic acid, and finally to convert the acid group arising thereby totally or partially into a salt by reaction with a base.
These anionic polyurethanes are very suitable for the sizing of paper; they can be used both for surface sizing and also in beater sizing.
However, these anionic polyurethanes are sensitive to polyvalent cations, as they are present if hard water, for example, is used for processing or if enzymatically degraded starch is used. In the presence of such ions, problems may arise in sizing. Calcium and aluminum ions can be specified in this case as problematic polyvalent ions. These anionic polyurethanes also exhibit a certain sensitivity to acids, so that precipitation of the anionic polyurethane can occur especially in beater sizing starting at pH 6.5, so that the effectiveness of sizing is reduced.
Thus although numerous polyurethanes with an anionic structure are already known, there still is a need for anionic polyurethanes with improved properties, as well as for suitable production processes, especially however for anionic polyurethanes that are suitable as sizing agents for paper for highly diverse purposes.